"Truth is truth" means that truth is objective—there are some things that are always right (and therefore antithetical things that are always wrong) such as 2 + 2 = 4, or the sun is a ball of hot plasma/burning gas. Without truth there is no knowing. "Christian truth" is a way of saying that Christians agree that truth is objective, and that the Bible (and Christian doctrine) are true. Truth is God's nature, and is intrinsic to the universe. Therefore science is truth, reasoning can be reliable, knowledge is possible, and truth is attainable

The foundational characteristic of the universe is "fact". The things that exist truly exist. There is an actuality to matter, energy, physical laws, coexistent relationships, forces, etc. Our whole notion of science is that things are true, that they can be found out, and we can count on them being true. We can observe them, and test them, and repeat the tests, because underlying all of it is something true. Now, the Bible says that the universe was created by Jesus, is a reflection of his nature, and that he sustains it all by his nature. That would mean that Jesus is the truth on which the universe is based (John 14.6), as far as Christians believe. This is "Christian truth," and it's just as objective as philosophical reasonings about truth.